1) Field of the Invention
The present invention relates to an optically controlled device, a method of controlling the optically controlled device, a spatial light modulator using the optically controlled device, and a projector using the spatial light modulator.
2) Description of the Related Art
Tilt mirror devices are used for spatial light modulators of projectors. The tilt mirror device includes a plurality of movable mirrors formed on an integrated circuit board based on micro electromechanical systems (MEMS). The movable mirror moves its reflecting surface according to an image signal, and reflects an illumination light. The tilt mirror device reflects the illumination light according to the image signal by controlling the movable mirrors, and modulates the illumination light. The technology for this tilt mirror device is disclosed, for example, in U.S. Pat. No. 5,867,202.
However, a conventional tilt mirror device includes an interconnect in order to drive the movable mirrors, and therefore, electrical access to the movable mirrors is required. Furthermore, the conventional tilt mirror device includes movable mirrors formed on an integrated circuit board using complementary metal oxide semiconductors (CMOS) or the like. Therefore, the conventional tilt mirror device has a complicated structure, and moreover, the integrated circuit and the MEMS structure have to be integrally formed, which causes reduction in yield. Thus, manufacturing costs increase.
An optically controlled device can be used for the tilt mirror device. The optically controlled device is driven by incident light. If the optically controlled device is used, the tilt mirror device can be controlled by scanning a light beam (hereinafter, “a control light”) according to an image signal onto each of the optically controlled devices (light addressing). By using the optically controlled device for the tilt mirror device, there is no need to provide the interconnect or the like for electrical access to the movable mirrors. Therefore, there is also no need to integrally form the integrated circuit and the MEMS structure, which allows improvement in yield.
Moreover, the movable mirror can be easily upsized, and therefore, it is also easy to realize high resolution. Since the integrated circuit is not needed, cost of the tilt mirror device can be reduced and the tilt mirror device can be driven without any restriction by a withstand voltage of the integrated circuit. Furthermore, since the integrated circuit is not required and a glass material can be used for the substrate, the tilt mirror device can be upsized at low cost.
However, the optically controlled device possibly used for the tilt mirror device has one side of tens of micrometers. Because the optically controlled device is such a micro device, it is necessary to illuminate a control light onto the optically controlled devices with high precision in order to control the tilt mirror device according to image signals. It is also necessary to set a spot diameter of the control light to about 10 micrometers in order to make the control light incident on an electrode of the optically controlled device.
On the other hand, in order to display an image, it is necessary to scan the control light at high speed. Scanning the control light at high speed makes it difficult to cause the control light to be precisely incident on the optically controlled devices. Furthermore, even if the control light can be scanned with high precision, a scanning speed of the control light becomes slower, which makes it difficult to display a high quality image. Therefore, precise control for the optically controlled device is difficult. As explained above, although there are some advantages such that the cost can be reduced by using the optically controlled device for the tilt mirror device, it is difficult to precisely control the optically controlled device.
The optically controlled device controls electrostatic force generated caused by a potential difference between an electrode for driving (hereinafter, “a driving electrode”) and a movable mirror that is a movable unit, and drives the movable mirror. If there is a slight amount of the potential difference between the driving electrode and the movable unit, the electrostatic force generated is also a slight amount. The slight amount of the electrostatic force generated between the two cannot sometimes drive the movable mirror. If there is a slight variable range of the potential difference between the two, only by making a slight amount of control light incident on the movable mirror, the potential difference between the two largely changes within the variable range.
Therefore, it becomes difficult to move the movable mirror to a precise position by changing the amount of control light. As explained above, the optically controlled device has some troubles because it becomes difficult to perform precise control over the movable mirror according to image signals if the potential difference between the two is a slight amount.